Labyrinth for unidirectional microphone

ABSTRACT

A labyrinth structure for a unidirectional ribbon-type microphone comprises an outer shell-like member and at least one inner member, each having open and closed ends, the space enclosed by the outer member being divided into at least two communicating chambers by the inner member. The inner chamber, in addition to being the end of the labyrinth path, can be used to house microphone components therein and also to shield these components by making at least one member of a good electrical conductor.

United States Patent [72] Inventor John C. Bleauy Trenton, NJ. [21] Appl. No. 786,265 [22] Filed Dec. 23, 1968 [45] Patented Nov. 9, 1971 [73] Assignee RCA Corporation [54] LABYRINTH FOR UNIDIRECTIONAL MICROPHONE 7 Claims, 5 Drawing Figs.

|52| U.S.Cl ..179/l38 VL [51 lnt.C1 .H04r 9/08 501 Field of Search 179 121 D,

l 56 1 References Cited UNITED STATES PATENTS 2,870,856 1/1959 Olson etal 181/.5 2,751,441 6/1956 Olson et 179/1155 2,566,094 8/1951 Olson etal 179/1155 2,352,305 6/1944 Anderson 181/33 2,271,988 2/1942 Olson 179/1 1545 2,102,736 12/1937 Olson 179/138 2,032,389 3/1936 Anderson 179/ l 38 Primary Examiner-Kathleen H. Claffy Assistant Examiner-Tom D. DAmico AnameyEdward J. Norton ABSTRACT: A labyrinth structure for a unidirectional ribbon-type microphone comprises an outer shell-like member and at least one inner member, each having open and closed ends, the space enclosed by the outer member being divided into at least two communicating chambers by the inner member.

The inner chamber, in addition to being the end of the labyrinth path, can be used to house microphone components therein and also to shield these components by making at least one member of a good electrical conductor.

LABYRINTH FOR UNIDIRECTIONAL MICROPHONE This invention relates to a unidirectional microphone of the type employing a ribbonlike conductor moveable in a magnetic field in response to sound waves directed against one side thereof and, particularly, to such a microphone which is smaller than has heretofore been available.

Microphones of this type require a labyrinth structure on the other side of the ribbon for acoustically terminating the sound waves passing the ribbon. Ideally, to acoustically terminate these sound waves, a pipe of infinite length on the other side of the conductor is required. In practice, however, since this is not possible, a long finite pipe or labyrinth path equivalent to a long finite pipe is used. The pipe or labyrinth may be filled with acoustic damping material to simulate a load equivalent to a termination path of infinite length. If a straight pipe is used, the microphone becomes large, bulky and cumbersome to handle. This is shown by U.S. Pat. No. 2,032,389 issued to L. J. Anderson in which the support stand for the microphone is utilized to house the labyrinth structure. To shorten the overall microphone length while still retaining a terminating pipe of suitable length, various folded labyrinth structures have been disclosed in patents to L. J. Anderson, U.S. Pat. No. 2,178,216; H. F. Olson, U. S. Pat. No. 2,271,988; H. F. Olson et al., U.S. Pat. No. 2,870,856; and A. R. Morgan, U.S. Pat. No. 3,383,476.

Basically, all of the above-disclosed labyrinths, although differing in specific structure, have as common features relatively large labyrinth structures and an additional microphone cavity extending beyond the labyrinth structure to house microphone components, such as an output transformer. Further, in order to.prevent unwanted sound pickup in the microphone output, the additional microphone cavity must house shielding for these microphone components. Consequently, the additional space requirements for the transformer and the like necessitate a larger, heavier, bulkier microphone than is desirable for such specialized applications as for use on a lectern, for example. In this case, a microphone of the least obtrusiveness is desirable to preclude excessive obstructions between a speaker and his audience.

It is therefore the object of the present invention to provide a miniature microphone of the type indicated having an improved labyrinth structure of relatively small size and which is capable of housing other microphone elements therein.

It is another object to provide an improved labyrinth structure made of a good electrical conductor for shielding any microphone elements contained therein.

It is still another object to provide a miniature microphone of the type indicated which, although of general use, is especially suitable for lectern applications.

In the present invention, the labyrinth is constructed from an outer shell-like member having an open end coupled to the ribbonlike conductorand a closed end. The open end may be similar to the closed end, except that is has an aperture therein. Further included is at least one inner member also having an open end and a closed end. The inner member may have the same shape as the outer member and is located within the outer member in spaced relationship therewith to divide the space enclosed by the outer member into at least two communicating adjacent chambers of which only one is bounded by both the outer member and the inner member. Each respective member has its open end spaced from and facing a closed end of an adjacent member to provide a continuous tortuous path through all of the chambers. Means are provided which are secured to the members for maintaining the chambers in fixed relationship with respect to each other.

In one form, the shell-like members may be cylindrically shaped like a common can. Also, the ends thereof may be orthogonal to the longitudinal axis of the member. In addition, the longitudinal axes of the members may be coaxial. In this form, the members may be nested within one another, dividing the space enclosed by the outer member into continuous, communicating chambers. The innermost chamber, in addition to being part of the labyrinth, may also be utilized to house microphone elements. By making at least one of the members out of a good electrical and magnetic conductor, shielding is provided for any microphone elements housed therein. A good acoustic resistance material such as polyurethane foam may be added to further reduce the required length of the labyrinth path.

Further advantages and objects of the present invention will be more readily apparent from the following description when taken in conjunction with the accompanying drawings, wherein: a

FIG. I is a side elevation of one form of a microphone according to this invention.

FIG. 2 is a central longitudinal cross section view taken on the line 2-2 of FIG. 1.

FIG. 3 is a simplified cross section view of only the labyrinth on line 2-2 of FIG. 1.

FIG. 4 is a plan cross section view of the labyrinth shown in FIG. 3 on line 44.

FIG. 5 is a partially exploded and cutaway perspective view of the labyrinth of which FIG. 3 is a cross section view.

Referring to the drawings, in FIG. 1, there is shown a microphone having two cylindrical shell-like cases Ia and I joined together to form a completed microphone case. At the top of the microphone there is disposed asemispherical screen 2 having a plurality of relatively small apertures therein for providing a passage means for sound waves to enter into the microphone. Just below the top of the microphone is screen 15 which is wrapped around case 1 covering holes 22 therein. This screen is of similar material as screen 2 and also provides passage means for sound waves to enter the microphone. Screen 2 is cemented in a retaining groove at the top of the upper case 1 while screen 15 is cemented in a groove in the side of .the case. The upper case 1 and lower case Ia are joined at the microphone midsection. This is accomplished by threading the two cases together by means of respective internal and external mating threads. The upper and lower cases are made preferably of a light weight material such as Dural.

In the upper housing case 1 there are enclosed the transducer microphone elements required to convert sound energy to electrical energy. Basically, there is a ribbonlike conductor 4 disposed in airgap 16 between two pole pieces 3 by means of a nonmagnetic clamp 38 secured to the two pole pieces 3. The pole pieces 3 are each respectively cemented to a respective top surface of two opposing magnets 5. Cemented to the bottom surfaces of the two opposing magnets 5 is bottom plate 7. Externally threaded retaining ring 36 is threaded into the internal threads of the upper case 1 to lock the above elements tightly together. Not shown are the conductive leads connected to both ends of the ribbonlike conductor. Between the two opposing magnets 5 there is centrally located labyrinth connector 8. Labyrinth connector 8 is here shown made partially of one piece molded RTV rubber 6 having aperture 9 located therein. Sound passed by holes 22 enters aperture 9 providing the required phase shift to make the response of the microphone unidirectional.

Labyrinth connectors are generally well known in the art. They are more specifically disclosed by U.S. Pat. No. 2,600,787 issued to H. F. Olson et al. and by U.S. Pat. No. 2,301,638 issued to H. F. Olson. The molded rubber 6, in addition, serves the function of holding magnets S in predetermined fixed spaced relationship, which is an advantage over the prior art. The labyrinth connector 8 is substantially elongated and rectangular in shape at the upper portion thereof similar to the shape and area of the ribbon 4 disposed immediately thereabove. The connector gradually tapers inwardly in a plane parallel to line 2-2 of FIG. 1 as seen in FIG. 2 and tapers outwardly in a plane normal to that plane. It then terminates in a substantially circular cross section area at the lower portion thereof located at hole 17 in plate 7. In connector 8 there is a central longitudinal hole 10 in the molded RTV rubber portion 6 for passing the conductor leads, not shown, from the ribbon conductor to transformer 13 disposed in lower case la. The hole 17 in bottom plate 7 is a continuation of and part of the labyrinth connector 8.

The front surface of the ribbon 4 is open for the reception of sound waves passing through the screen 2. These waves cause the ribbon to vibrate in the magnetic field of magnets 5. Thus, there is generated sound representing signal voltages in conduction leads, not shown, connected to the ribbon ends. The conduction leads are coupled to transformer 13 located in lower case 1a. The output of the transformer is then coupled to utilization means, not shown. In the embodiment of FIG. 2, the various elements are made of the following materials. The ribbon is preferably corrugated aluminum while the ribbon clamp 38 can be brass insulated from the pole pieces 3 by fish paper not shown. The magnets 5 are preferably Alnico V-7 which has a higher energy product than prior magnets as noted in the Morgan Patent. This results in a smaller magnet for a given flux density further reducing the size of the microphone. The bottom plate 7 can be made of armco iron as can be the top pole pieces 3. Any other suitable material can also be used for these parts.

The labyrinth structure is housed primarily in the lower case la. When case lais threaded into upper case 1, it tightly clamps outwardly extending flange 30 of outer member 11 of the labyrinth structure against pliable compressable ring of rubber 37 commonly known as an O-ring. The O-ring 37 is thereby compressed against the lower surface of retaining ring 36, sealing member 11 to plate 7. The bottom surface of plate 7 coupled with the inner surface of retaining ring 36 and O- ring 37 form a relatively continuous sealed surface comprising the outer boundary of the initial labyrinth region 18. The bottom plate 7 forms part of the labyrinth path as well as part of the labyrinth connector in the embodiment shown.

The retaining ring 36 and O-ring 37 in the labyrinth structure of FIG. 2 have been deleted from FIG. 3 to clarify the details of the structure. Referring now to FIG. 3, the labyrinth structure has 'an outer member 11 shaped like a cylindrical shell with an open end 38 and opposing closed end 25. End 25, as shown here, is substantially parallel to end 38. Open end 38 has flange 30 extending outwardly from the edge of the member orthogonal to the longitudinal axis of the outer member 11. Flange 30 is required only to secure outer member 11 to bottom plate 7 and is not required for the labyrinth of this invention. Other means, of course, could be provided for securing outer member 11 to plate 7. Bottom plate 7 has a hole 17 therein which passes the sound waves initially in the direction of arrow 23 into the labyrinth structure of FIGS. 3 and 5. Bottom plate 7 is a washerlike piece secured to the outer member 11 providing a continuous sound-impermeable surface from hole 17 to region 19. The inner surface of member 11 forms the outer boundary for the initial labyrinth path.

The inner boundary for the initial labyrinth path is formed by member 12. Member 12 is shaped like a cylindrical shell having an open end 24 and a closed end 31. This member is spaced in coaxial relationship with member 11. Open end 24 of member 12 is disposed in spaced relationship from and facing closed end 25 of member 11. At the opposite side, closed end 31 of member 12 is disposed spaced from and facing open end 38 of member 11 and plate 7. Member 12 has its outer diameter and length somewhat smaller than the respective inner diameter and length of member 11. Thus, when member 12 is disposed within member 11, the structure consists of a member within a member creating communicating adjacent chambers defined by the walls thereof forming the labyrinth passageway. The open and closed ends of the members at ends 38 and 25 of the labyrinth form transverse regions for continuous communication and coupling of adjacent longitudinal regions. Member 12 has at its open end 24 an inwardly extending flange 26 parallel to closed end 31. End 24 forms with closed end 25 of member 11 region which is in continuous communication with region 19 which is, in turn, in communication with region 18 and which regions are bounded by members 11 and 12, and by member 12 and plate 7, respectively. Members 11 and 12 are both relatively thin walled. Member 11 may be made of 0.025 inch thick cold-rolled steel and member 12 may be made of 0.025 inch thick copper, both of which are good electrical conductors. Member 12 is maintained in fixed spaced relationship to member 11 by securing means such as screws 14 of FIG. 2. The screws 14 secure member 12 in fixed relationship to plate 7. Since member 11 is also secured to plate 7, as previously described, members 11 and 12 are therefore secured in fixed relationship to each other. Member 12 has therein an inner chamber 40 which can be larger than regions 18, 19 and 20, which together form the outer chamber. Disposed within inner chamber 40 of member 12 can be microphone elements such as transformer 13. Transformer 13 forms the inner boundary and member 12 forms the outer boundary of regions 21 and 29 of chamber 40. Region 29 is in continuous communication with region 20 by means of aperture 32. Transformer 13, which is substantially enclosed by a good electrical conductor consisting of members 12 and 11, is, therefore, shielded from stray and unwanted electrical fields.

The sound waves to be terminated initially enter the labyrinth by way of labyrinth connector 8 shown by arrow 23. Once the waves enter the transverse labyrinth region bounded by plate 7 and member 12, they diverge radially in the transverse direction. Arrows 39 of FIG. 5 show this. The waves then propagate longitudinally normal to the direction of arrows 39 in the direction of arrows 34 in the outer region bounded by members 11 and 12. FIG. 4 shows a cutaway plan view of the labyrinth of FIG. 3 in direction 4-4. This figure may be referred to for further clarification of the labyrinth path. Tails of arrows representing the direction of the sound waves are shown by crosses, and heads of arrows are shown by dots within a circle. The sound waves upon reaching the lower extremity of longitudinal labyrinth region 19 enter transverse region 20. There they converge radially in the transverse direction communicating continuously with region 29 by means of aperture 32. The waves diverge radially in region 29 in the transverse direction to region 21 communicating continuously therewith and travelling therein in the longitudinal direction shown by arrows 35..By this time the waves in the desired frequency range have been terminated.

To provide acoustical damping, a suitable acoustic resistance material 33 is placed within the central portion 10, not shown, of labyrinth connector 8, and throughout the labyrinth regions of FIG. 3. A preferred acoustic material is open-pore polyurethane foam. This can be used in the fabrication of multiple microphones to produce consistent performance characteristics between units more easily than heretofore has been possible with prior materials. In addition, it is unaffected by moral ranges of temperatures and humidity. This is in contrast to Ozite which is widely used for acoustic resistance and which is less uniform and which requires a higher degree of care in installation in order to achieve consistent performance between different microphones. The foam 33 can be preformed to fit in the regions bounded by member 12 and transformer 13 and can, therefore, provide the means for securing the transfonner within the inner region of member 12. This further reduces the number of heavier and bulkier fastening devices utilized in the prior art to secure the transformer to the microphone. The foam has the further ad'- vantage of being capable of being preformed to fit the various labyrinth regions. This provides simpler and faster manufacturing than prior materials.

Sound waves enter the microphone by passing through screen 2 impinging upon ribbon conductor 4. Those waves passing by that conductor enter airgap l6 behind the conduc tor. Sound waves also enter aperture 9 by means of screen 15 and holes 22 giving the microphone its unidirectional charac teristics. The waves then continue through the labyrinth connector 8. The sound, clamped by rubber portion 6 and foam portion not shown, continues to interconnected hole 17 in plate 7, which is also filled with acoustic resistance material 33. Upon emerging from hole 17, the sound then enters the labyrinth in region 18. There the sound continues as described above, damped by acoustic resistance 33, until it terminates in the inner chamber 40 of member 12. The inner chamber containing transformer 13 is characterized by regions 29 and 21 as previously described. The sound by this time is terminated as if it has travelled in a pipe of infinite length.

It should be noted that region 18 bounded by member 12 and plate 7 may be created by substituting for plate 7 an inwardly extending end 38 on member 11 similar to the portion 26 of member 12, by one skilled in the art, without deviating from this invention. Also, flange 30 on member 11 is utilized for assembly purposes only and is not part of this invention.

The foregoing description reveals the present invention to be advantageous over the prior labyrinths by utilizing the concept of spaced shells. Not only are the shells light and easily assembled, but they permit greater space utilization within the microphone. This results in a smaller and lighter unidirectional microphone than possible with those labyrinths existing heretofore. This unidirectional microphone is useful on lectems where small, unobtrusive microphones having good performance characteristics are needed.

The description herein explains the preferred embodiment for the most practical application. It is apparent, however, to one skilled in the art, that shells in spaced relationship may take a variety of forms without deviating from the present invention. In the embodiment shown, two cylinderlike shells are utilized, but, of course, a greater number could be provided to further increase the labyrinth length as may be required by particular microphone applications. Also, cylinders are here shown in coaxial spaced relation to each other having holes in portions of one end thereof. Of course, a labyrinth could be provided in which the two outermost cylinders have opposing closed ends serving concurrently as closed ends for further inner cylinders. These inner cylinders could project altemately, finlike, in meshed relationship, from the opposing closed ends of the two outermost cylinders. These and other arrangements could easily be devised by those skilled in the art, and being more complex than the arrangement shown herein, are omitted for clarity.

WHAT IS CLAIMED IS:

l. In a microphone of the type having a vibratory conductor in a magnetic field, a labyrinth structure for providing an acoustical termination path for said conductor comprising:

an outer member in the form of a shell having an open end coupled to said conductor and a closed end, and at least one inner member having an open end and a closed end located within said outer member for dividing the space within said shell into at least two communicating adjacent chambers only one of which is bounded by both said outer member and said one inner member, said members including portions partially closing the open ends thereof, said portions each having a hole therein, means secured to said members for maintaining said chambers in fixed relationship with respect to each other, and

each respective member having said open end thereof spaced from and facing a closed end of an adjacent member to provide a continuous tortuous path through said chambers.

2. The invention according to claim 1, wherein said members comprise cylindrical shells having their respective axes parallel to each other, said open and closed ends being parallel to each other.

3. The invention set forth in claim 2, wherein said cylindrical shells are located in coaxial spaced relation, and said open and closed ends being orthogonal to the longitudinal axis of said coaxial shells.

4. The invention according to claim 1, wherein at least one of said members is fabricated from material of good electrical conductivity whereby any components situated within said member are electrically shielded.

5. The invention of claim 2 wherein said cylindrical shells are nested in spaced relation to each other.

6. The invention of claim 1 characterized by the addition of acoustic resistance material in said continuous acoustical termination path.

7. The invention of claim 6, wherein the acoustic resistance material is polyurethane foam. 

1. In a microphone of the type having a vibratory conductor in a magnetic field, a labyrinth structure for providing an acoustical termination path for said conductor comprising: an outer member in the form of a shell having an open end coupled to said conductor and a closed end, and at least one inner member having an open end and a closed end located within said outer member for dividing the space within said shell into at least two communicating adjacent chambers only one of which is bounded by both said outer member and said one inner member, said members including portions partially closing the open ends thereof, said portions each having a hole therein, means secured to said members for maintaining said chambers in fixed relationship with respect to each other, and each respective member having said open end thereof spaced from and facing a closed end of an adjacent member to provide a continuous tortuous path through said chambers.
 2. The invention according to claim 1, wherein said members comprise cylindrical shells having their respective axes parallel to each other, said open and closed ends being parallel to each other.
 3. The invention set forth in claim 2, wherein said cylindrical shells are located in coaxial spaced relation, and said open and closed ends being orthogonal to the longitudinal axis of said coaxial shells.
 4. The invention according to claim 1, wherein at least one of said members is fabricated from material of good electrical conductivity whereby any components situated within said member are electrically shielded.
 5. The invention of claim 2 wherein said cylindrical shells are nested in spaced relation to each other.
 6. The invention of claim 1 characterized by the addition of acoustic resistance material in said continuous acoustical termination path.
 7. The invention of claim 6, wherein the acoustic resistance material is polyurethane foam. 